1. Field of the Invention
This invention relates to the field of infrared (IR) photodetection. More particularly, the invention relates to semiconductor IR detectors that are capable of being tuned by an applied bias for multicolor detection.
2. Description of the Prior Art
The use of layering semiconductor materials to form a superlattice heterostructure to act as a photodetector is known. U.S. Pat. No. 4,894,526 issued to Bethea et al on Jan. 16, 1990 and entitled, "Infrared-Radiation Detector Device," discloses a narrow-bandwidth, high-speed IR detector that is based on tunneling of photo-excited electrons out of quantum wells. Such photodetectors have also been designed for wide-band operation. Examples of wide-band IR detectors are disclosed in U.S. Pat. No. 5,013,918 issued to Choi on May 7, 1991 and entitled, "Multi-Color Infrared Photodetector," and my U.S. patent application Ser. No. 867,726, filed Mar. 23, 1992 and entitled, "Wide-Range Multicolor IR Detector," both of which are incorporated herein by reference.
Further, the use of these wide-band IR detectors for multicolor detection is also known. One existing multicolor IR detection system comprises a wide-band IR detector and a rotating mechanical multicolor filter wheel. The wide-band-detector detects any wavelength of IR radiation falling on the detector. The rotating filter wheel selects the desired wavelength to pass through and illuminate the detector. Although this system has served the purpose, it has not proved entirely satisfactory for a number of reasons. These systems have been found to be bulky, heavy, slow and subject to mechanical failure. Additionally, because some incident IR is filtered out, these systems are usually insensitive.
For IR detection technology to find widespread commercial and military applications many of these problems must be overcome. For example, to be more acceptable, IR aircraft guidance systems must be able to see through clouds better in different weather conditions. It is known that optimum IR detection wavelengths depend on weather conditions. Specifically, different humidity levels attenuate IR radiation of different wavelengths to a different degree. As such, a multicolor IR detector that can easily switch to different detection wavelengths would be highly useful in this area.
A multicolor IR detector in a satellite survey system provides much useful information about the Earth's natural resources. Different minerals and vegetation reflect IR radiation of different wavelengths to a different degree. Multicolor IR photos of such natural resources may be used to identify the quantities and species of target materials, analogous to viewing the various colors of different objects under visible light.
In like manner, a multicolor IR detector can provide useful information on geographical surveys. For instance, manmade structures appear more prominently in a natural landscape when a two-color detector is used. Also, the temperature of the ocean can be measured more accurately with a multicolor IR detector.
Multicolor IR detector systems have great potential use as medical diagnostic tools, making better thermal images of a human body than other like systems. In summary, multicolor IR detectors find important uses in the following fields: aircraft, mining, agriculture, aerospace, fishing and medical.